1. Field of the Invention
This invention pertains to a depth measurement method and a depth measurement apparatus for measuring depth or step difference of a depression in a pattern in which depressions and protrusions periodically repeat, and is specifically suited for use, e.g., when measuring the depth or step difference of fine groove patterns, such as the trench structure or alignment marks in a semiconductor element, and the like.
2. Related Background Art
For example, semiconductor elements are formed by layering multi-layered circuit patterns in specified positional relationships onto a wafer, and in the lithographic process for forming the circuit pattern for each layer, the pattern image of a reticle, which acts as a mask, is transferred onto a photoresist layer on the wafer at a specified magnification by an exposure device. And, when the reticle patterns are transferred onto the second and subsequent layers of the wafer by the exposure device, the location of the alignment marks formed on the wafer in the preceding processes is detected, and once the reticle pattern and the pattern already formed on each shot field on the wafer have been aligned with each other based on the results of this detection, so-called "overlay exposure" is performed.
The alignment marks on past wafers have usually been patterns in which depressions and protrusions periodically repeat formed on the wafer at a specified pitch along the direction of measurement. Below, these three-dimensional patterns which are thus periodically arranged in a specified direction will also be referred to as "groove patterns", and the fine patterns with small three-dimensional pitches within these groove patterns will be referred to as "fine groove patterns." When detecting alignment marks comprising this kind of fine groove pattern with, e.g., a photographic alignment sensor which uses an illumination light of a specified bandwidth, the parts in the image of the alignment mark which correspond to the boundaries between the protrusions and depressions may be a dark line, for example, making it possible to detect the dark line. There are also alignment sensors which perform position detection based on diffracted light in a specific direction from the fine groove pattern, such as the laser step alignment type (LSA type), which shines a light beam, such as a laser beam, onto alignment marks comprising a diffraction grating-shaped fine groove pattern, detects the diffracted light from that pattern and perform position detection based on the intensity of the diffracted light, or the two-beam interference type (LIA type) which shine coherent light beams from two directions onto the pattern, and perform position detection based on the phase of a beat signal obtained by photoelectrically converting a pair of diffracted lights exiting parallel to each other from the pattern.
For example, when performing position detection in this way, based on light diffracted in a specific direction from the fine groove pattern in this way, the intensity of the emitted diffracted light varies greatly, and in some cases the SN ratio of the resulting detection signal deteriorates, depending on the depth of the pattern depressions, i.e., with the step difference between the depressions and the protrusions. Consequently, it is necessary to control the manufacturing processes so that the depth of these patterns remains within a specified range, and for that it is necessary to precisely measure the depth of the patterns.
In addition, when it is necessary to precisely control the depth of a groove pattern other than alignment marks on a wafer, it is assumed that it will be necessary to measure their depth accurately. From the past, the method used for measuring the depth (or step difference) of a fine groove pattern has been to shine light fluxes of multiple wavelengths onto and around that fine groove pattern and calculate the depth from the spectral distribution of the normal reflected light.
This exploited the fact that reflected light from the bottom (depression) of the fine groove pattern and the reflected light from the upper surface (its surroundings) would interfere with each other, and that the reflected light of specific wavelengths would be stronger or weaker due to the phase difference which accompanies the difference in the light paths of the two reflected lights (2.times. the depth).
However, there was a problem in the prior art as described above in that the SN ratio of the obtained signal was not very good since detection was performed dependent on the intensity of the reflected light from the depressions in the groove pattern and from the are surround them. There was also the problem that, when the material of the groove pattern is one which gives rise to variations in the reflecting power within the wavelength band of the detection light, the variations in reflecting power for a wavelength peculiar to a material had an effect on the variations in reflecting power that accompanied the aforementioned depth variations, drastically reducing the accuracy of depth detection.